scaling up 35mm

I was wondering how much I can scale up 35 mm in post so that it still looks good on the big screen? The 35mm was 3perf and will be transfered to HD SR.
I was told I could scale up to 110%, is this true?
I would only scale up certain shots in my edit, will the difference with the other shots be noticable?
Would there be a difference if we did a 2k scan instead of a HD scan?

You will get much much better results if you scale up during telecine, rather than during the edit. I'm not sure how much you can go at that point, but it would be more than 110%. Probably much more, and it still look good (whether or not it would match is a different story). If you want to go that route, the best person to talk to would be the actual colorist who is taking care of your telecine... he or she will know exactly how much they can blow it up and stay within acceptible-looking limits.

The 110% is probably a good guess for blowing up HD that has already been transfereed full frame. More than that and it's going to start to look ratty, and you are not going to notice a huge size difference with 110% anyway.

Yes, if you get a 2K scan instead of HD you will have a much greater range for potential enlarging.